If There's Life on Alien Planet Gliese 581g, How Do We Find It?

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After spending
decades searching for alien planets capable of harboring life,
astronomers may have found one. So how can they check to see if life
actually exists on this alien world?

Yesterday (Sept.
29), a team of researchers announced the discovery
of Gliese 581g, a rocky, roughly Earth-size
planet in its parent star's so-called
"habitable zone" a just-right range that can allow
liquid water to exist.

One of the planet's
discoverers said in a briefing yesterday that "the chances of
life on this planet are 100 percent." To determine if this is
true, researchers will have to scrutinize Gliese 581g from afar,
searching its atmosphere for certain telltale molecules.

But it might be a
while before they have the tools to do this properly.

Listening for
signals

Gliese 581g isn't
far from Earth in the great scheme of things only 20.5
light-years or so. But that translates to about 120 trillion miles
(194 trillion kilometers) 500 million times farther away from us
than the moon. [ Tour
the six Gliese 581 planets.]

So human-built
probes won't be getting out there anytime soon. But one way to look
for life on Gliese 581g is to turn our radio telescopes toward the
planet, searching for patterns in emissions of electromagnetic
radiation.

Such patterns could
indicate the presence of intelligent life, according to Seth Shostak
of the SETI (Search for Extraterrestrial Intelligence) Institute in
Mountain View, Calif.

The Gliese 581 star
system has intrigued researchers for a while, so they've already
taken a few looks. During SETI's Project Phoenix, which surveyed
almost 1,000 star systems from 1995 to 2005, astronomers looked at
Gliese 581 twice, Shostak said.

"No signal was
found during these observations," he told SPACE.com.

What's in your
air?

Life doesn't have to
be intelligent and advanced for astronomers to pick it up. Studying
Gliese 581g's atmosphere, for example, could theoretically reveal the
presence of organisms as simple as microbes.

This method assumes
the alien planet has an atmosphere, likely a necessity for life to
take hold. Gliese 581g's discoverers reported that the planet's
gravity is probably strong enough to hold onto an atmosphere, but
they didn't definitively detect one.

"The first
thing is, you've got to have an atmosphere," said Bill Borucki
of NASA's Ames Research Center, the science principal investigator
for NASA's planet-hunting
Kepler mission. "If there is one, then
what's the composition of that atmosphere?"

If astronomers
detect the signatures of large, complicated compounds like
chlorofluorocarbons, which people have manufactured to use as
refrigerants and propellants life is likely, according to
Borucki.

"You typically
don't have both gases present in significant quantities unless life
is present," Jenkins told SPACE.com. But scientists would have
to be careful how they interpreted such information, he added,
because we don't know much about how other planetary systems tick.

"Though
scientists get excited about these discoveries just like the public
does, we also tend to be pretty cautious," Jenkins said.

Tough job for
today's tech

So scanning Gliese
581g's atmosphere, if it has one, would give us a good idea if the
planet harbors life or not. But it'll probably be a few decades
before we can do this properly.

Astronomers have
characterized the atmospheres
of alien planets before. But those other worlds
are bigger and much hotter, meaning they throw off lots of radiation
for our instruments to pick up. Gliese 581g is relatively close to
Earth, but its other traits make it a tough read.

It's only about
three to four times as massive as Earth, for example, with an average
surface temperature between minus 24 and minus 10 degrees Fahrenheit
(minus 31 to minus 12 degrees Celsius).

"Because they
are cool and small, planets like this are very difficult to study,"
Jenkins said. "It's easier to detect something than to
characterize it in detail."

Jenkins said that
Gliese 581g also apparently doesn't transit its parent star, meaning
it doesn't cross in front of it from our perspective on Earth.
Astronomers can learn a lot about a planet's atmosphere by studying
starlight that passes through it but this technique is likely not
an option with Gliese 581g.

As a result, the
tools astronomers currently have at their disposal likely can't
determine what's in Gliese 581g's air, according to Borucki and
Jenkins. So researchers will have to wait for new instruments to come
into play.

A few decades
away

One promising tool
mentioned by Borucki, Jenkins and Shostak is NASA's proposed
Terrestrial Planet Finder (TPF) mission, which would use an array of
telescopes orbiting Earth to generate detailed images of alien
planets.

TPF would employ
advanced techniques to reduce the glare of the exoplanets' parent
stars, allowing the mission to pick up faint radiation coming from
planets. The mission could theoretically detect chemicals like
methane and oxygen in the atmospheres of alien worlds such as Gliese
581g.

The TPF mission,
however, is in limbo. It is currently unfunded, with no launch date
set. So researchers will probably have to wait a while before they
can see what Gliese 581g's atmosphere is made of.

Whenever TPF, or
something like it, comes along, it may have a long list of planets to
check out, Jenkins said.

"I would
predict that [Gliese 581g] is just the tip of the iceberg," he
said. "Fifteen or 20 years ago, very few people thought we'd be
discovering such extrasolar planets anytime soon. This find just
shows how far we've come."